On the use of mechanical and acoustical excitations for selective heat generation in polymer-bonded energetic materials
To address security issues in both military and civilian settings, there is a pressing need for improved explosives detection technologies suitable for trace vapor detection. In light of the strong dependence of vapor pressure on temperature, trace vapor detection capabilities may be enhanced by selectively heating target materials by external excitation. Moreover, polymer-bonded energetic materials may be particularly susceptible to heating by mechanical or acoustical excitation, due to the high levels of damping and low thermal conductivities of most polymers. In this work, the thermomechanical response of polymer-based energetic composites and methods for acoustical excitation are investigated in order to improve the understanding of the temperature rises induced by applied excitation, and to uncover waveforms which may efficiently transmit excitation energy to generate heat and enhance trace vapor detection capabilities. The heat generation in the binder material of energetic and surrogate systems under harmonic excitation was investigated analytically through the application of a viscoelastic material model. Specifically, structural-scale heating was considered under low-frequency direct mechanical excitation as applied to a beam geometry. Experiments were conducted with a mock mechanical material, wherein the mechanical and thermal responses were recorded by scanning laser Doppler vibrometry and infrared thermography, respectively. Direct comparisons between the model and experimental results demonstrated good agreement with the predicted response, with low-order, bulk-scale heating observed along the modal structure in areas of higher strains. In addition, localized heating near individual crystals was investigated analytically by extending the viscoelastic heating model to general three-dimensional stress-strain states. Application of the model to a Sylgard 184 binder system with an embedded HMX (octogen) crystal under ultrasonic excitation revealed predictions of significant heating rates, particularly near the front edge of the crystal, due to the wave scattering and the resulting stress concentrations. In considering methods for such excitation through incident acoustical or ultrasonic waves, the form of the wave profile was tuned in this work for the purpose of maximizing the energy transmission into solid materials. That transmission is generally limited by the large impedance mismatch at typical fluid--solid interfaces, but by varying the spatial distribution of the incident wave pressure, significant transmission increases can be achieved. In particular, tuned incident inhomogeneous plane waves were found to predict much lower values of the reflection coefficient, and hence large increases in the energy transmission in the context of lossless and low-loss dissipative media. Also, material dissipation was found to have a strong effect on the optimal incident waveform, generally causing a shift to lower inhomogeneity values. Similar results were obtained for parameterized forms of bounded incident waves with respect to the local reflection phenomena and surface wave excitation. These results suggest that, depending on the targeted solid material, substantial energy transmission and heat generation increases may be achieved by tailoring the spatial form of the incident wave profile.
Bolton, Purdue University.
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